Many FM403 Fantic 300 Professional twin-shocks do not run anywhere near as well as
they can do, following re-packing of the front pipe, and modification of the early
type port timing to the later specification.

The FM403 front pipe appears to be a pressed steel expansion chamber, in common with
the FM450 and FM350. This is in fact not the case, as the pipe on the 249cc bike
includes a passive silencing element , which can be re-packed after the pipe has
been cut open.

FM403 front pipe ready for re-packing

Click to enlarge

Classictrial modified FM403

Click to enlarge

Fantic 300 Professional set up

* Finally please remember that all of the above information is provided on the strict
proviso that any modifications or changes not carried out specifically by Classictrial,
are not guaranteed or warranteed in any way, and are entirely the responsibility
of the individual carrying them out, and that Classictrial are not responsible in
any circumstances for damage or injury that may occur as a result of poor workmanship
*

Properly re-packing the front pipe on an FM403 means much improved running, with
smoother power delivery, and also seems to make bikes grip a little better. The ideal
exhaust set up for these bikes is a re-packed stock front pipe, in combination with
a passive straight through back box.

The porting on the FM403 was designed to work with the old 5 star leaded fuel, and
the position of the exhaust port is such that detonation/knocking will often occur
when the motors are highly loaded at low rpm. The later 249cc motors exhaust porting
configuration was altered, and this cured the detonation problem and provided increased
performance at the same time.

We can modify FM403 cylinders to later spec, but it is quite possible to fit a later
mono-shock cylinder (301,303) to the earlier twin-shock bottom end, and this is a
very useful modification that makes a noticeable difference to performance. The very
last of the 300 Professionals produced, the “Michaud” replica's, were fitted with
a cylinder which had the late type porting arrangement, so these don't need to be
changed obviously.

Finally we can assist with repacking FM403 front pipes fitted to machines being
modified by us, but cannot help otherwise, as this is a time consuming and dirty
job, and most people are unwilling to pay for this type of work to be carried out
professionally!